1. Field of the Invention
The present invention relates to a line loss handling method and apparatus in an optical transmission system, and in particular, to a method for controlling an automatic laser shutdown (ALS) function upon detection of a line loss.
2. Description of the Related Art
An automatic laser shutdown (hereinafter referred to as ALS) function is generally used to automatically shut the laser down in order to protect the eyesight of a worker who repairs a defective optical line between optical repeaters or between an optical repeater and a terminal station in an optical transmission system.
For example, in an optical transmission system exemplary of contemporary practice in the art an optical line between optical repeaters has a line loss, with the optical repeaters having working lines and protection lines in either direction, respectively. In this regard, in a method exemplary of contemporary practice in the art of performing an ALS function specified by ITU-T Recommendation, it is assumed that a first optical line which is a working line in a direction from a second optical repeater to a first optical repeater has the line loss. Upon detection of the line loss on the defective optical line, the first optical repeater determines whether the line loss state continues for a preset time of 550 ms, for example. If the line loss state continues for the preset time 550 ms, the first optical repeater performs line switching to shut down the defective working line. After the line switching, the first optical repeater designates an optical line in an opposite direction of the defective optical line and performs the ALS function for the designated optical line. That is, upon detection of the line loss, the first optical repeater performs the ALS function for an optical line in the opposite direction if the line loss state continues for the exemplary preset time of 550 ms.
Here, the method exemplary of contemporary practice in the art of designating the optical line in the opposite direction can be classified into two methods: the first method exemplary of contemporary practice in the art is to designate a working line in the opposite direction if the defective optical line is the working line and designate a protection line in the opposite direction if the defective optical line is the protection line; and the second method exemplary of contemporary practice in the art is to match the optical lines on the one-to-one basis such that, for example, first and second optical lines are fixedly matched with third and fourth optical lines, respectively.
A description will be made as to an operation of designating the optical line in the opposite direction according to the first method exemplary of contemporary practice in the art and then performing the ALS function. It is assumed that the first and fourth optical lines are set to the working lines W and the second and third optical lines are set to the protection lines P.
Upon detecting a defect on the first optical line (which is the working line) between the second and first optical repeaters, the first optical repeater switches the defective first optical line before performing the ALS function. That is, upon detection of the line loss, the first optical repeater switches the first defective optical line having the line loss from a working state to a protection state before the passage of the exemplary preset time of 550 ms. After the line switching, the first optical repeater performs the ALS function for the protection third optical line in the opposite direction of the switched first optical line to shut down the laser.
Next, a description will be made as to an operation of designating the optical line in the opposite direction according to the second method exemplary of contemporary practice in the art on the assumption that the first and second optical lines are matched with the third and fourth optical lines, respectively, and then performing the ALS function. Upon detecting a defect on the working first optical line between the second and first optical repeaters, the first optical repeater performs the ALS function for the working third optical line matched with the working first optical line to shut down the laser if the line loss on the defective working first optical line continues for the exemplary present time of 550 ms. In the meantime, during the shutdown of the laser, the third and fourth optical lines switch their operating states. Here, in the case that the third optical line is the working line, the third and fourth optical lines make an unnecessary switching, which can cause a deterioration of the system performance.
As the first optical repeater performs the ALS function according to the first or second method exemplary of contemporary practice in the art as stated above, the second optical repeater then recognizes the line loss through the protection third optical line. Subsequently, the second optical repeater shuts down the laser by performing the ALS function for the optical line corresponding to the protection third optical line.
That is, in recognizing the line loss by the first method exemplary of contemporary practice in the art, the ALS function is performed for the protection first optical line paired with the protection third optical line. Also, in recognizing the line loss by the second method exemplary of contemporary practice in the art, the ALS function is performed for the first optical line matched with the third optical line.
As stated above, however, the second method exemplary of contemporary practice in the art can cause an unnecessary switching of the defect-free working third optical line and the defect-free protection fourth optical line, which can deteriorate the transmission performance of the optical transmission system. In addition, assume that the first optical line is in a lock-out state due to its previous transmission state. Here, the lock-out state refers to a state in which the line switching is not performed. That is, the first and second terminal stations perform a compulsory command to prevent the line switching by the internal affairs (e.g., the repeated line switching for a specified time) and the control option, and the state caused by performance of this command is referred to as the lock-out state. For example, if the first terminal station issues a lock-out command to prevent the line switching from the first optical line to the second optical line, the first optical line continues to be a working line and the second optical line continues to be a protection line, until the lock-out state is released. In the lock-out state, the operation result according to the first method exemplary of contemporary practice in the art does not turn out as expected.
The ALS function exemplary of contemporary practice in the art in the optical transmission system which is set to the lock-out state is described as follows. If the first and second terminal stations issue the lock-out command to prevent the switching from the first optical line to the second optical line, the first optical line continues to be a working line serving as a primary channel and the second optical line continues to be a protection line serving as a secondary channel, until the lock-out state is released. Although the first optical repeater should switch, upon detection of the line loss, the defective first optical line having the line loss from the working state to the protection state before the passage of the exemplary preset time of 550 ms, it cannot perform the switching operation because the first optical line continuously maintains the working state. Accordingly, the first optical repeater performs the ALS function for the working fourth optical line in the opposite direction. In the meantime, since the opposite direction is not in the lock-out state, the switching is performed is within 550 ms, for example. Accordingly, even in this case, the third and fourth optical lines are unnecessarily switched, thereby deteriorating the transmission performance of the optical transmission system. The second optical repeater then detects the line loss through the fourth optical line, and upon detection of the line loss, undesirably performs the ALS function for the protection second optical line in the opposite direction to shut down the laser output from the second optical line. Accordingly, although the laser output from the defective first optical line should be shut down, the laser output from the normal second optical line is shut down unexpectedly.
U.S. Pat. No. 4,739,162 to Ortiz Jr. et al., entitled Laser Beam Injecting System, discloses a system for injecting successive beam pulses of a pulsed power laser into optical fibers, for transmission therethrough. It is disclosed that the system comprises four lens, four fiber groups with the tips thereof respectively proximate the focal points of the lenses, two galvanometer driven mirrors for directing the laser beam through one of the lenses for focusing onto a selected fiber tip, and control means to drive the galvanometers to reorient the two mirrors between laser beam pulses into successive pairs of predetermined positions effective to inject the successive beam pulses into selected fibers. Failure to reorient the mirrors before the succeeding beam pulse arrives results in laser shutdown. Means are provided for detecting a malfunction of the mirrors, fracture of a lens or a missed fiber injection and, in either case, shutting down the laser.
U.S. Pat. No. 4,812,641 to Ortiz Jr, entitled High Power Optical Fiber Failure Detection System, discloses that a break or leak in an optical fiber transmitting high power laser energy, at average power levels sufficient for material processing, is detected promptly and the laser beam delivery system shut down when the optical fiber begins to fail. It is disclosed that photo detectors monitor the laser power out of the fiber and injection power into the fiber, in particular the light intensities in the fiber input and output couplers. A difference in detector outputs, larger than a set threshold to account for inherent fiber losses, is an indication that a break or leak has occurred.
U.S. Pat. No. 5,428,471 to McDermott, entitled Fail-Safe Automatic Shut-Down Apparatus And Method For High Output Power Optical Communications System, discloses that in a fiber-optic communications system, a shut-down apparatus in the event of a fiber-optic cable disruption includes a first optical fiber cable for propagating signals in a first direction and having a plurality of adjacent amplifiers disposed along the first cable. It is disclosed that a second optical fiber cable for propagating signals in a second direction, opposite the first direction, includes a plurality of adjacent amplifiers disposed along the second cable. Each of the amplifiers in the second optical fiber cable are interconnected to one of the amplifiers of the first optical fiber cable to form a plurality of amplifier pairs. Circuitry is provided for terminating operation or reducing the output power level of an amplifier within the first or second optical fiber cables in the event of a cable disruption between adjacent amplifier pairs, such that an amplifier within an amplifier pair adjacent to the cable disruption terminates generation or reduces its power level to a safe level at its output. Circuitry is further provided for generating a continuity signal on the first and second cables at the output of each of the amplifiers. Circuitry is provided for sensing the continuity signal upon elimination of the cable disruption to thereby actuate the input of the amplifier within an amplifier pair adjacent to the cable disruption to thereby automatically reestablish communication along the previously disrupted cable.
U.S. Pat. No. 5,771,114 to Andersson, et al., entitled Optical Interface With Safety Shutdown, discloses a system for transmitting optical power from a first location to a second location. The system is disclosed to include a first light source at the first location which generates a first light beam. A power converter detects the first light beam at the second location. A first control circuit coupled to the power converter operates a second light source at the second location to generate a return safety light beam after detection of the first light beam. A photodetector detects the return safety light beam at the first location. A second control circuit is coupled between the photodetector and the first light source. The second control circuit detects the presence of the return safety light beam and operates the first light source to generate the first light beam at a first power level prior to detecting the return safety light beam and at a second power level, higher than the first power level, after detecting the return safety light beam.
U.S. Pat. No. 5,822,112 to Itou et al., entitled Control Apparatus For Optical Amplifier, discloses a control apparatus for an optical amplifier such as an erbium-doped optical-fiber (EDF) amplifier suppresses overshooting of ALC control at power on and at the time of input light restoration from an off condition, while ensuring quick starting. At power on and at the time of input light restoration from an off condition, an idling current (IDC) reference value generating circuit generates an IDC reference voltage that increases to a value sufficiently greater than a value in a normal operating condition with a time constant corresponding to a rise time of the EDF, and thereby controls a laser diode for pumping the EDF. The voltage can be set to a value larger than the value in the normal operating condition for a predetermined period of time by using a timer.